The present invention relates generally to digital systems employing multi-level pulse width modulation (PWM), and more specifically to digital audio systems employing multi-level PWM to generate audio output signals from digital input signals.
U.S. patent application Ser. No. 10/819,573 filed Apr. 7, 2004 entitled MULTI-LEVEL PULSE WIDTH MODULATION (PWM) IN DIGITAL SYSTEM (the '573 application) discloses a digital system that employs PWM and other digital signals to control switching circuitry in multiple channels for generating an analog output signal from a multi-bit digital input signal. The analog output signal can be an audio sound output or any other suitable type of analog output. In one embodiment, the digital audio system controls the switching circuitry in a fixed one of the multiple channels using a PWM signal and a plurality of other digital signals, in which the PWM control signal is generated from a predetermined number of least significant bits (LSBs) of the digital input signal, and the other digital control signals are generated from the rest of the most significant bits (MSBs) of the digital input signal. The system controls the switching circuitry in the remaining channels using only the digital control signals generated from the MSBs of the digital input signal. As a result, the fixed channel provides a multi-level PWM signal that can be taken as the sum of a single-level variable-width pulse signal (the variable-width component) and a multi-level maximum-width pulse signal (the multi-level component) with a maximum pulse width at its output, while the remaining channels provide multi-level maximum-width pulse signals at their respective outputs. The signals provided at the outputs of the respective channels are typically low pass filtered before being provided to loudspeakers for producing the desired audio sound. It is noted that the maximum pulse width of a multi-level PWM signal corresponding to a PWM signal representing a first digital data value with a fixed number of binary digits, the first digital data value being the LSBs of a second digital data value represented by the multi-level PWM signal, is the theoretical width of the PWM signal when the first digital data value is increased by 1 beyond its maximum value. This theoretical width is herein referred to as the maximum pulse width reference, the maximum permissible pulse duration, or the maximum pulse duration or width of the corresponding PWM signal.
One drawback of the digital audio system disclosed in the '573 application is that when the level of the digital input signal varies from slightly above to slightly below a given level or vice versa, the PWM signal controlling the switching circuitry in the fixed channel can undergo a significant change in width, ranging from near zero to near the maximum pulse width or vice versa. This change in the width of the PWM control signal can cause a corresponding change in the variable-width component of the multi-level PWM signal provided at the output of the fixed channel. In addition, while the variable-width component of the multi-level PWM signal undergoes such a change in width, one or more of the multi-level signals provided by the remaining channels can exhibit a significant level variation. In an ideal system, such changes in the widths and/or the levels of the output signals provided by the various channels can offset one another, resulting in little or no detrimental effect on system performance. However, because the channels in a practical system can have different performance characteristics, such changes occurring in the output signals of the various channels can cause unwanted transient outputs, which are often very noticeable to the system user.
It would therefore be desirable to have a digital audio system employing multi-level PWM that avoids the drawbacks of the above-described digital audio system.